An existing Passive Optical Network (PON) providing a point-to-multipoint optical fiber access is typically applied in the field of a broadband access to provide a user with communication services of data, e.g., voice, video, etc. Referring to FIG. 1, a typical PON system includes an Optical Line Terminal (OLT), an Optical Network Unit (ONU) and an Optical Distribution Network (ODN). A signal is transmitted between the OLT and the ONU over the optical distribution network including no active device but composed of passive optical devices, e.g., optical fiber, optical splitters, etc. A signal is transmitted through a single bidirectional fiber in the PON system typically with an uplink signal wavelength of 1310 nm and a downlink signal wavelength of 1490 nm for the ONU and an analog video data wavelength of 1550 nm.
A Distributed NodeB is a predominant type of station for a 3G NodeB, and a fundamental principle thereof lies in that respective functional modules of the NodeB are deployed at different physical locations using remote technology and a core module typically includes a Base Band Unit (BBU) and a Remote Radio Unit (RRU). Referring to FIG. 2, the BBU may be connected with a plurality of RRUs to improve the efficiency of sharing a baseband pool. In actual deployment, BBUs may be arranged centrally to conserve a resource of station addresses. This application generally relates to transmission of a baseband signal between the BBU and the RRU, which demands a high bandwidth, low time delay and jitter of time and frequency, thus the BBU and the RRU are connected typically through fiber straight drive.
Since PON is generally adopted for broadband access, the signal is generally transmitted to a metropolitan area network; and transmission between the BBU and the RRU of the distributed NodeB is generally connected through fiber straight drive, thus the baseband signal is terminated between the BBU and the RRU. Since source and destination addresses and transmission requirements between the broadband access and the baseband signal transmission are different, it is difficult to share an optical fiber and a transmission system. In order to improve the efficiency to utilize the optical fiber access devices, it can be considered that the signal transmitted over the Passive Optical Network (PON) and the baseband signal transmitted in the distributed NodeB be transmitted collectively over the same optical fiber access network. However, the purpose of collective transmission can not be achieved due to the presence of the following problems in the prior art:
1. In the existing PON system, a transmission rate 1 Gb/s of an uplink signal is below a total required bandwidth between the BBU and the RRU in the distributed NodeB and thus fails to satisfy the demand for transmission of the baseband signal in the distributed NodeB;
2. The BBU and the RRU are connected through fiber straight drive, and uplink and downlink signals are transmitted through different optical fibers, thus wasting a resource of optical fibers and lacking manageability;
3. Time and frequency synchronization is highly demanded in a TDD system, e.g., TD-SCDMA, etc., and if transmission in the distributed NodeB is performed with the traditional method, then an introduced jitter of time and frequency may grow linearly with the increasing number of the cascaded RRUs, thus making it more difficult to perform time and frequency synchronization; and
4. An available communication bandwidth per RRU on average may drop linearly with the increasing number of the cascaded RRUs; the average available bandwidth may be improved with a transmission system at a high rate, but an investment on transmission devices will be increased greatly.